Langton’s ant is a two-dimensional universal Turing machine with a very simple set of rules but complex emergent behaviour. It was invented by Chris Langton in 1986 and runs on a square lattice of black and white cells. The universality of Langton’s ant was proven in 2000. The idea has been generalized in several different ways, such as termites which add more colours and more states.
Modes of behavior
These simple rules lead to complex behavior. Three distinct modes of behavior are apparent, when starting on a completely white grid.
1). Simplicity, during the first few hundred moves it creates very simple patterns which are often symmetric.
- Chaos. After a few hundred moves, a large, irregular pattern of black and white squares appears. The ant traces a pseudo-random path until around 10,000 steps.
- Emergent order. Finally the ant starts building a recurrent “highway” pattern of 104 steps that repeats indefinitely.
All finite initial configurations tested eventually converge to the same repetitive pattern, suggesting that the “highway” is an attractor of Langton’s ant, but no one has been able to prove that this is true for all such initial configurations. It is only known that the ant’s trajectory is always unbounded regardless of the initial configuration – this is known as the Cohen–Kung theorem.
In 2000, Gajardo et al. showed a construction that calculates any boolean circuit using the trajectory of a single instance of Langton’s ant. Additionally, it would be possible to simulate an arbitrary Turing machine using the ant’s trajectory for computation. This means that the ant is capable of universal computation.